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SPEAKER EFFICIENCY : AN INITIAL REVIEW by Peter W. Mitchell
F o r m a n y p e o p l e b u y i n g s t e r e o e q u i p m e n t , t h e q u e s t i o n o f w h e t h e r t o s p e n d a n a d d i t i o n a l $ 5 0 - $ 1 0 0 t o g e t a 6 0 - w a t t r a t h e r t h a n 30-watt amplifier is an important one. Yet few people consider the fact that the choice of loudspeaker can completely offset that doubling of amplifier power. How many audiophiles are aware, for example, that KLH-17s driven by a 20-watt amplifier can play louder than Small Advents driven by a 60-watt amp ? If, like many listeners, you never play music louder than a 90-db sound pressure level, then this
difference is irrelevant; any speaker on the market can be driven comfortably to 90 db in a living room by a good 20-watt-per-channel
So you can select loudspeakers on the basis of their sound
character, bass range, size, appearance, and cost. But if you want to reproduce the 100-db levels of a full orchestra in a concert hall or
a grand piano in a salon, you ought to consider the speaker's efficiency as well as its other parameters, or else be prepared to buy a big amp.
Probably the reason why people don't consider loudspeaker efficiency carefully is that manufacturers have made useful data on
relative efficiency hard to obtain.
The Standards Committee of the
Institute of High Fidelity, which is responsible for defining how manu-
facturers and test labs should measure the performance parameters of high-fidelity equipment, has standardized the specifications of
amplifiers and is currently revising its FM tuner specification standards, but has never standardized any speaker measurement techniques, not even for speaker efficiency. So manufacturers don't advertise efficiency, magazine test reports have not generally measured efficiency, and the buying consumer is left in the lurch. Of course the IHF is a manufac- turers' association, not a consumer-oriented body, and the IHF Standards
Committee operates at the manufacturers' bidding. There is some disagreement among industry members about how best to measure speaker performance parameters, and apparently the IHF would rather leave consumers completely ignorant than specify tests which might produce a misleading result once in a while.
However there are four ways in which at least approximate values of relative loudspeaker efficiency can be obtained. I have used all of these in compiling the attached table.
(1) Since mid-1970 High Fidelity's speaker test reports have included response measurements in an anechoic chamber, taken all around the speaker at a microphone distance of 1 meter and processed with a, small computer to produce three response curves. One of these is the on-axis frequency response, but the curve of interest here is the solid line representing the speaker's total acoustic power output, for an electrical input of one watt, averaged over all directions. To repre- sent the speaker's efficiency, I take the average output in the range from 100 Hz to 2 kHz (where most of the acoustic power in music occurs).